Method and apparatus for batch, active alignment of laser arrays to fiber arrays

ABSTRACT

An apparatus for aligning a substantially co-linear array of optical devices with a substantially co-linear array of optical fibers comprising; a carrier having a slot for receiving the array of devices and the array of optical fibers; a first support plate having a first holder for holding the array of devices in place with respect to the first support plate, and electrical connections for electrically connecting to selected ones of the devices in the array of devices; a second support plate having a second holder for holding the array of fibers in rough alignment with the array of devices; and a positioner for positioning the carrier with respect to the first holder and the second holder so that when the holders are activated, the array of devices and the array of fibers are held in approximate alignment without the presence of the carrier.

TECHNICAL FIELD OF THE INVENTION

This invention relates to first-level packaging of transmitting opticalinterconnects. More particularly, it relates to a method and apparatusfor actively aligning a one-dimensional array of edge-emitting diodelasers to a corresponding array of single-mold fibers.

BACKGROUND ART

In optical communications, it is difficult and often costly to make theconnection between laser diodes and fibers, particularly single-modefibers. In order to achieve efficient optical coupling, the laser mustbe aligned to the fiber to tight tolerances; typically ±1 to 2 μm forsingle-mode fibers and ±10 μm for multimode fibers.

Methods of laser-to-fiber alignment may be divided according to twocriteria:

1. Piece-by-piece vs. batch. In piece-by-piece methods, an individuallaser die, usually pre-packaged in a TO can, is aligned to a singlefiber, or to the center of a bore which later accepts a fiber centeredin a precision ferrule. By contrast, in batch methods, an array oflasers is aligned to an array of fibers. This succeeds to the extentthat each array is straight and the center-to-center spacings match.

2. Active vs. passive. In active alignment methods, the laser isenergized to produce light; whatever portion of this light successfullyemerges from the back end of the fiber is monitored by a photodetector.Thus, the best-aligned position is determined explicitly by moving theenergized laser with respect to the fiber to maximize the photodetectedsignal. By contrast, in passive alignment methods, the best-alignedposition is determined implicitly by microscopic visual alignment ofcertain geometric features assumed to have known relationships to theoptical centerlines.

Most prior-art alignment schemes are piece-by-piece and active. Theseschemes are inherently expensive because each part is handledindividually, each fiber is individually polished, and each alignment isseparately performed. Moreover, arrays of interconnects which are neededto achieve high-speed data rates via multiple fibers are bulky andinconvenient if built from piece-made packages.

Several prior-art alignment schemes are batch and passive. Batchalignment is advantageous because it shares the cost of handling,polishing, and alignment among the many elements of an array. However,the advantages of passive alignment are questionable: the schemesrequire costly precision jigging for "dead reckoning," and impose tighttolerances on the location of fiducials or other geometric featureswhich must be precisely located with respect to the optical centerlines.Moreover, the method may require custom laser arrays having specialfiducials, and imposes stringent tolerances on the V-grooves which holdthe fibers. Both of these requirements are costly.

SUMMARY OF THE INVENTION

It is a principal object of the invention to provide a method andapparatus which combines the cost advantages of batch alignment with thecost advantages, simplicity and dependability of active alignment.

The present invention is directed to a method for aligning asubstantially co-linear array of lasers with a substantially co-lineararray of optical fibers, comprising the steps of activating a laser inproximity to a first end of said laser array; positioning the laserarray with respect to the fiber array to maximize energy coupled fromthe activated laser to its corresponding fiber; activating a laser inproximity to a second end of said laser array; and positioning the laserarray with respect to the fiber array to maximize energy coupled fromthe activated laser to its corresponding fiber. The method may furthercomprise activating at least one additional laser in said laser array;positioning the laser array with respect to the fiber array to maximizeenergy coupled from the additional laser to its corresponding fiber; andusing regression analysis to determine the optimum position of the laserarray with respect to the fiber array to optimize the coupling inaccordance with a predetermined criteria.

One possible criteria may be a least squares fit. Alternatively, thecriteria includes positioning the laser array with respect to the fiberarray so that the energy which represents the minimum amount of energycoupled between a laser and its respective fiber is maximized.

The method further comprises the step of positioning at least onephotodetector to detect light coupled into the fiber. The photodetectoris used to determine the maximum energy coupled from an activated laserto a corresponding fiber.

The present invention also contemplates an article of manufacturecomprising a substantially co-linear array of lasers; a substantiallyco-linear array of optical fibers substantially in alignment with saidarray of lasers; a substrate for supporting said array of optical fibersand said array of lasers; and a plurality of relatively large electricalcontact pads, each pad being electrically coupled to one of said lasers,said pads being for receiving electrical contacts for activatingselected ones of said lasers.

The present invention is also directed to an apparatus for aligning asubstantially co-linear array of lasers with a substantially co-lineararray of optical fibers comprising; a carrier having a slot forreceiving the array of lasers and the array of optical fibers; a firstsupport plate having a first holding means for holding said laser arrayin place with respect to said first support plate, and means forelectrically coupling to selected ones of said lasers; a second supportplate having a second holding means for holding the fiber array in roughalignment with the laser array; and means for positioning said carrierwith respect to said first holding means and said second holding meansso that when said holding means are activated, said laser array and saidfiber array are held in approximate alignment without said carrier.

The holding means may each comprise means for supplying a vacuum, and anopening in a respective support plate through which vacuum is applied toa respective array.

The apparatus may further comprise adjustment means for moving saidfirst support plate with respect to said second support plate; and anoptical microscope for observing alignment of said laser array withrespect to said fiber array.

The apparatus may additionally comprise positioning means forpositioning a substrate to which said laser array and said fiber opticarray are to be bonded, in close proximity thereto. The positioningmeans may include a window positioned so that electromagnetic energy maybe transmitted through said substrate.

The window and substrate are preferably transparent to ultraviolet lightso that ultraviolet curing adhesive applied to said substrate to bondsaid substrate to said laser array and said optical fiber array may becured by said ultraviolet light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view of a package produced inaccordance with the invention.

FIG. 2 is an enlarge perspective conceptual view used to illustrate thealignment method of the invention.

FIG. 3 is an enlarged scale fragmentary perspective view of a portion ofthe apparatus of FIG. 1.

FIG. 4(a) is an enlarged perspective view of an apparatus for holdingthe laser assembly in accordance with the invention.

FIG. 4(b) is an enlarged perspective cut away view of the bottom of theapparatus of FIG. 4(a).

FIG. 5 is an enlarged perspective view of an apparatus for holding afiber assembly.

FIG. 6 is an enlarged perspective view illustrating the means fordelivering the laser and fiber arrays to the alignment apparatus.

FIG. 7 illustrates the central elements of the alignment apparatus afterdelivery of the laser and fiber arrays.

FIG. 8 is a perspective view of the entire alignment apparatus, with themicroscope omitted to allow enhanced viewing.

FIG. 9 is an enlarged perspective view of a fixture used to deliver theglass substrate and cure the adhesive.

FIG. 10 is a typical graphical representation of coupling efficiency asa function of misalignment for various values of laser-to-fiberseparation.

FIG. 11 is a graphical representation comparing batch-aligned couplingefficiency with optimized, individually aligned coupling efficiencyacross an array of thirty two laser-fiber pairs.

FIG. 12 is a graphical representation of the ratio of couplingefficiencies across an array of thirty two laser-fiber pairs.

FIG. 13 is a graphic representation of power fluctuations during thegluing process.

FIG. 14 is a graphical representation of a comparison of couplingefficiencies before and after gluing across an array of thirty twolaser-fiber pairs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 depicts a final package P produced by the method and apparatus ofthe invention. Package P includes a laser assembly 1 a fiber assembly 2,and a substrate 3 to which both assemblies are bonded after alignment.Laser assembly 1 consists of a laser bar 4 which is soldered andwire-bonded to a metallized silicon carrier 5. The fiber assemblyconsists of an array of stripped, single-mode fibers 6 which are epoxiedto a V-grooved silicon carrier 7 and polished on both ends. The presentinvention includes a method of aligning the two assemblies and attachingthem permanently to the substrate 3.

FIG. 2 illustrates, conceptually, the alignment scheme. First, the twoarrays are aligned coarsely by visual means under microscope objective8. Then, active alignment is used for the critical fine tuning. Inpreparation for active alignment, the laser bar 4 has been soldered tothe silicon carrie 5 via the center gold pad 9 (a common ground pad),and two lasers 10 and 11, one near each end of the array, have been wirebonded to the side pads 12 and 13 via wire bonds 14 and 15. Duringactive alignment, the three pads are connected to a current source 16via switch 17, so that the two selected lasers 10 and 11 may beenergized alternately. The fiber sub-assembly 2 is positioned in frontof a wide-area photodetector 18, so that the light transmitted througheither of the two fibers, 19 or 20, impinges on the photodetector,thereby producing a signal at output leads 21. The size of thephotodetector is large enough to capture the light from both fibers 19and 20 without mechanical repositioning. Thus to monitor alternately thealignment of the two laser/fiber pairs 10, 19 or 11, 20, it is necessarymerely to toggle the switch 17. With this arrangement, active alignmentis achieved simply by micropositioning the laser assembly with respectto the fiber assembly to maximize the photodetected signals.

FIG. 3 illustrates details of the laser bar 4. It is a thirty-two-laserarray of 1.3-μm wavelength lasers spaced on 375-μm centers. The laserbar 4 and its silicon carrier 5 are drawn to scale in this figure.Dimensions G, H and I are 0.3 mm, 13.5 mm and 0.12 mm, respectively. Inall other figures, only five of the thirty-two lasers are shown for thesake of clarity, and the size of the laser bar is greatly exaggeratedwith respect to other objects.

Laser bar 4 has a common, back-side ground (not shown) which is a goldpad covering the entire bottom surface of the laser bar. As discussedpreviously (FIG. 2), this gold pad is soldered to the center pad 9 ofthe silicon carrier, while the "hot" side of several selected lasers arewire bonded to the side pads 12 and 13. Although conceptually only twobonded-out lasers are necessary (as suggested in FIG. 1 and FIG. 2), itis useful in practice to bond out two additional lasers, at intervalsalong the array, as shown in FIG. 3. Thus there are five gold pads 9,12, 13, 22, 23, and the current source 16 in FIG. 2 is connected to afour-pole (rather than a two-pole) switch. This permits the alignment tobe assessed at several intermediate points along the array, rather thanjust at the ends, which is useful in checking on collinearity of thearrays. The gold connection pads are relatively large. By large it ismeant that the pads have dimensions larger than that of the lasers and asize which facilitates electrical connection thereto during thealignment procedure.

The configuration in FIG. 3 is merely illustrative. More generally, thearrangement of gold pads and wire bonds is dependent on the laser bar'sdesign and on package requirements. For example, for most realisticconfigurations, laser-driver circuitry would be mounted on the siliconclose to the lasers themselves. Furthermore, for high-speed arrayinterconnects, it may be preferable for each laser to have a separateground return. Such variations are entirely compatible with theinvention, and do not alter its essence.

FIG. 4(a) and 4(b) illustrate an apparatus for holding the laserassembly. To permit attachment of the common substrate 3 shown in FIG.1, it is necessary, during alignment, to leave the bottom surface of thelaser carrier 5 unobstructed. Thus it is necessary to hold the laserassembly, and to make electrical connections to the five gold pads, fromabove.

Laser carrier 5 is held to the bottom surface 25 of plate 24 by means ofthe vacuum chuck 26, which is fed via a vacuum fitting 27 such as aClippard Minimatic Fitting. The surface 25 is ground and polished flatto prevent warping the laser assembly when vacuum is applied. This isnecessary to avoid spoiling the collinearlity of the lasers along thebar, which is an essential requirement for batch alignment. Afterpolishing, the surface is made electrically insulating (by applicationof 100 Å of Ti followed by 3000 Å of SiO₂) to prevent shorting theseveral contacts to each other. The force of the vacuum chuck issufficient to compress springs (not shown) urging spring-loaded pins 28,commonly known as "pogo pins" (made by Interconnect Devices as modelTRI-0-C-1-1N) so that the five gold pads on the laser carrier 5, pulledflush to the surface 25, make good electrical connection to the fivecompressed pins.

The pins 28 extend through the thickness of plate 24. Theirnon-spring-loaded ends 29 protrude through the top surface. Each pin issurrounded by an insulating collar 30, made of Vespel polyimide, whichis push fit into a hole in the steel plate. As shown in FIG. 4(a), thetops of the pins 29 make contact to pieces of insulated wire 31 with theinsulation being stripped back near the pins as shown. The strippedwires are butted to the pins and soldered. The insulated portion of thewires 31 lay snugly in slots 32 in the steel plate, and emerge near therear of the plate at 33. Here the insulation is also stripped back, sothat the five wires form the male end of a standard, 0.1"-pitchconnector. Thus the electrical connections suggested conceptually inFIG. 2 are realized in a context which leaves the bottom surface of thelaser assembly unobstructed and capable of accepting the commonsubstrate 3 after alignment.

FIG. 5 illustrates an apparatus for holding the fiber assembly, fromabove, for the reason already discussed. In addition, to permit activealignment, the fibers must be monitored by the large-area photodetector18 (FIG. 2). In FIG. 5, the fiber assembly 2 is held to plate 34 byvacuum, which is supplied through vacuum fitting 35. The vacuum seal isimperfect because the centers of the fibers lie in the plane of thefiber carrier's top surface, so the gap between the fiber carrier andthe vacuum chuck is one fiber radius (62.5μ). Nevertheless, the vacuumis strong enough to hold the fiber assembly securely. The large-areaphotodector 18 is affixed to L-bracket 36, which locates the centerlineof the photodetector at roughly the same z-coordinate as the fibercores. Micrometer adjustment of photodetector 18 is provided in theother two directions, x and y, via stage micrometers 37 and 38,respectively. During alignment, photodetector 18 is moved close to thefibers to insure that all the light is captured; during final attachmentof the substrate 3 (FIG. 1), photodetector 18 is retracted. The vacuumplate 34 is designed to provide maximum stiffness within the workingdistance allowed by microscope objective 8 (FIG. 2). For this reason theplate is milled down at the front. Surface 39 is only 9 mm above thefibers, which is less than the working distance of the objective. Twoobjectives which may be typically used are Leica 569143, 10×, with aworking distance of 17 mm or Leica 569244, 25×, with a working distanceof 11 mm.

The alignment procedure begins by delivery of the two critical parts,the laser assembly 1 and fiber assembly 2, to the alignment fixture.FIG. 6 illustrates how these parts come together in the alignmentfixture.

Parts delivery is essentially a very coarse alignment procedure. Thelaser assembly 1 and the fiber assembly 2, are placed into the machinednotch 40 of delivery table 41, and their inside edges are aligned by eyeto the index marks 42 as shown. Next the delivery table 41 is slid inthe negative y direction, along the path of the dashed lines L, ontoplatform 43 of lifter stage 44 (which may be a Newport Model 416), usingnotch 45 as a guide. Notch 45, which runs the length of the bottomsurface of delivery table 41, is machined for a close, sliding fit overplatform 43. The sliding motion is arrested by contact of locating pins46 against the respective front planar surfaces 47A and 47B of vacuumplates 24 and 34 which have been previously aligned in any of severalways well known in the art. Finally, platform 43 is raised (+zdirection), using micrometer 48, thereby compressing the pins 28 (FIG.4) and bringing laser assembly 1 and fiber assembly 2 into contact withvacuum plates 24 and 34, respectively. At this point the vacuum chucksare turned on, via vacuum supplied hoses (not shown) connected tofittings 27 and 35, and the delivery table is lowered and removed. Theresult is shown in FIG. 7. The delivered parts are now firmly held tothe bottom surfaces of vacuum plates 24 and 34.

The sequence of steps just described accomplishes rough alignment of thetwo parts as follows: Rough alignment in the x,y, and θ directions isachieved by notch 40, locating pins 46, index marks 42, and the slidingfit of notch 45 on platform 43. This insures that the delivered partsmate properly with the vacuum chucks, and that the gold contact pads onthe laser assembly mate properly with the pins 28. Rough alignment inthe φ, ψ and z directions is achieved by indexing of the laser and fiberassemblies to the bottom surfaces of the vacuum plates 24 and 34. Thesetwo surfaces are coplanar by virtue of tilt adjustments described below.

FIG. 8 is an assembly drawing of the alignment apparatus A with themicroscope objective omitted for clarity. The apparatus is placed undera high-quality microscope (not shown) with a large working area such asLeica's ELR microscope stand. The entire assembly rests on ashock-isolated table, such as Newport VW-3042. At the top of FIG. 8 arethe vacuum plates 24 and 34 (described previously in FIG. 4 and FIG. 5),which hold the laser and fiber assemblies respectively. At the center ofthe figure is the lifter stage 44. (described previously in FIG. 6),which is used for parts delivery. The remainder of the equipment ispresent for the purpose of positioning the laser and fiber assemblieswith respect to each other and with respect to the microscope objective8 (shown in FIG. 7).

The positioning equipment may be regarded as four stacks of stages:

1. Master stages. Underlying the other three stage stacks are the masterxy stages 49 and 50 such as Melles Griot 07TAC012, which are used toposition the remainder of the assembly (everything above and including alarge base plate 51) with respect to the microscope objective in the xand y directions respectively. Master adjustment in the z direction ishandled by the microsocpe's focussing stage.

2. Laser stack. Located on the left of FIG. 8, the laser stack consistsof micro-stepped fine positioners including linear stages 52 and 53(Klinger UT100), a rotary stage 54 (Klinger UR100), and a goniometriccradle 55 (Klinger B650), which manipulate the laser assembly in the x,y, θ, and φ directions respectively, as well as a leveling assembly 56and 57. The linear stages 52 and 53 have 0.1-μm resolution(micro-stepped); the angular stages 54 and 55 have 0.001° resolution.The (φ, ψ) attitude of levelling plate 56 is controlled by three,fine-pitched, ball-bearing-tipped adjustment screws 58,59 and 60. Thesescrews are used for preliminary tilt adjustment of the laser-holdingvacuum plate 24, in order to establish parallelism of its bottom surfaceto the focal plane of the microscope in the φ and ψ directions. Thistilt adjustment needs to be done only once when the fixture is set up:subsequently, fine adjustment of the laser assembly in the φ directionis handled by the motorized stage 55; whereas fine adjustment in the ψdirection is not necessary, since laser-to-fiber alignment is not verysensitive in that direction.

3. Delivery stack. Located in the center of FIG. 8, the delivery stackconsists of a lifter stage 44, described earlier in connection with FIG.6, an auxiliary stage 64, which adjusts the position of lifter stage 44in the x direction, and a three-point levelling plate 65 (analogous toplate 56 described above), which is used to establish parallelism of thedelivery table 14 (FIG. 6) with respect to the focal plane of themicroscope. Again, this tilt adjustment needs to be done only once.

4. Fiber stack. Located on the right of FIG. 8, the fiber stack consistsof micro-stepped lifter stage 66 (such as a Klinger UZ100) and agoniometric cradle 67 (such as a Klinger B680) which manipulate thefiber assembly in the z and φ directions with resolutions of 0.1 μm and0.001° respectively. A three-point levelling plate 68 (analogous to theplate 56 discussed above) is used to render the bottom surface of thefiber-holding plate 34 parallel to the focal plane of the microscope inthe φ and ψ directions. Again, this tilt adjustment needs to be doneonly once.

The one-time tilt adjustments of levelling plates 56, 65, and 68 may beaccomplished quickly using a high-power microscope objective and eyeballassessment of focus. Since this tilt adjustment is done only once foreach plate (it is not repeated for each laser/fiber array), the manualnature of the adjustments is not an impediment to low manufacturingcost.

All three tilt adjustments are similar. As an example, the adjustment ofplate 56 is considered. The objective is to render the bottom of thevacuum plate 24 parallel to the focal plane of the microscope. To dothis, a surrogate piece of silicon or other flat material is held to thebottom of plate 24 (as the laser assembly is held in FIG. 4). Thesurface of the surrogate material is then a surrogate for the bottom ofplate 24 (the two surfaces are coplanar, yet the surrogate is visiblethrough the microscope). Thus the surrogate may act as the targetsurface for the levelling operation. A particularly simple levellingprocedure follows. This procedure works because the ball-bearing-tippedlevelling screws (58, 59 and 60) are arranged in an isosceles trianglehaving a base parallel to the y axis. It works only if performed in theorder given. Levelling in the y and x directions may not be reversed.

a. Using the focussing stage of the microscope (with digital readout ofheight), the height differential Δh_(y) is measured between two pointson the target surface having x values which are equal but having yvalues which differ by Δy.

b. Levelling screw 58 is adjusted by ##EQU1## where ΔY is the y distancebetween screws 58 and 60. The target surface is now parallel to the yaxis.

c. The height differential Δh_(x) is measured between two points on thetarget surface having y values which are equal but having x values whichdiffer by Δx.

d. Levelling screw 59 is adjusted by ##EQU2##

where ΔX is the x distance between screws 58 and 59. The target surfaceis now parallel to both the x and y axes.

After adjustment, the attitude of plate 56 is locked in place withlocking screws 61, 62 and 63, which are threaded into the lower plate57.

After the laser and fiber assemblies have been loaded into the alignmentfixture (FIG. 7), a coarse/fine strategy is used to align the twoarrays. Coarse alignment is purely visual (under the microscope), and isdone in the following order:

1. Align φ. For each of the two arrays, the master stage 50 is used tocompare focus at the two ends. Then, employing a method similar to thatdescribed above for one-time tilt adjustments of leveling plates 56, 65and 68, "fine-tuning" tilt corrections via goniometric stages 55 and 67is computed and applied.

2. Align z. Using the fine lifter stage 66, both arrays are brought intothe same focal plane.

3. Align θ. Using rotary stage 54, the front edge of laser assembly 1 isaligned to be parallel to the front edge of fiber assembly 2. Duringthis procedure, it may be necessary to use the x stage 52 to bring thearrays close enough together to make and accurate visual determinationof parallelism.

4. Set separation distance s. Using the x stage 52, a value of thelaser-to-fiber separation distance s (typically on the order of 35 μm)is selected.

5. Align y. Using the linear stage 53, one of the laser ridges isvisually brought into line with the center of the corresponding fiber.

The sixth degree of freedom, ψ, does not need fine adjustment; theinitial tilt adjustments suffice.

Fine alignment is done actively, using the photodetected signals fromthe four energized lasers, as described above with respect to FIG. 2 andFIG. 3. The procedure is quite simple and fast for several reasons:

1. The coarse-alignment described above is very satisfactory forbringing the two arrays nearly into alignment (typically close enough toget a signal from the photodetector immediately) which eliminates blindsearching.

2. In principle, only two degrees of freedom (y and z) are involved. Inpractice, slight adjustment of φ may also be desired to reach an optimalcompromise among the four energized lasers. Compromise is required onaccount of pointing-angle variations in the lasers, eccentricities ofthe fibers, imperfect collinearity of laser and fiber arrays, andimperfect center-to-center spacing. To find the best compromise, each ofthe four individual optima is found, a best-fit line is computed, andthe stages are indexed to accommodate it.

3. Optimization in the y and z directions is virtually independent,because coupled power as a function of misalignment is a smooth,single-peaked function. Thus it is typically necessary to find themaximum in each of the two directions only once and there is no need togo repeatedly back and forth between y and z. Even in a worst case, itis not necessary to perform more than three iterations (y--z--y orz--y--z).

After alignment has been optimized, the substrate 3 shown in FIG. 1 mustbe affixed to the underside of the aligned arrays, to lock thenpermanently in the aligned position. The method used to deliver thissubstrate is shown in FIG. 9. The method is similar to that describedearlier (FIG. 6) for delivering the laser and fiber assemblies. Thedelivery table 69 in FIG. 9 is analogous to table 41 in FIG. 6. Notch 70and locating pins 71 in FIG. 9 are analogous to notch 45 and pins 46 inFIG. 6. In FIG. 9, substrate 3 is a piece of glass such as a microscopeslide. The glass has a film of optical cement such as Summers J-91 onits top surface, which may be applied using a photoresist spinner. Afterpositioning the glass using the locating pins 71, the lifter stage 44(FIG. 6) is used to raise the glass into contact with thealready-aligned laser and fiber assemblies. As depicted by the largearrow in FIG. 9, the glue is then cured using ultraviolet light U. Thelight is incident at the tail of the arrow, is turned 90° by mirror 72,is transmitted through a cutout 73 and glass slide 3, and impinges onthe glue. Following a cure of 1 to 2 minutes, the vacuum at fittings 27and 35 is removed, and the finished assembly, shown in FIG. 1, isreleased onto table 69 and withdrawn from the alignment fixture.

Coupling efficiency (the fraction of laser light coupled into the fiber)is the key figure of merit for laser-to-fiber alignment. For reference,a typical plot of coupling efficiency as a function of misalignment, forvarious values of the laser-to-fiber separation s, is shown in FIG. 10.The measurement shown agree fairly well with theoretical predictions.

Coupling efficiency is inevitably compromised by batch alignment, asmentioned above, because it is never possible to align all thelaser/fiber pairs in an array as precisely as if each pair were adjustedseparately. Of course the trade-off is deliberate. Some efficiency issacrificed in exchange for the cost and packaging advantages of batchprocessing. A measure of just how much is discussed below.

For each laser/fiber pair, let

η_(b) ≡Batch-aligned coupling efficiency

η_(i) ≡Optimized, individually aligned coupling efficiency.

The values of η_(b) and Θ_(i) may be measured, prior to attaching thesubstrate 3, for each laser/fiber pair in an array. Typical resultsacross a thirty-two-wide array are shown in FIG. 11. Although there isquite a bit of variation from one laser/fiber pair to another, thisvariation is typically compensated in manufacturing by laser-trimmedresistors in the driving circuitry, so that the final output power isadjusted to a specified level.

To compare batch alignment and individual alignment directly, let##EQU3## Typical values of this ratio R, a figure of merit for batchalignment, are plotted in FIG. 12. The curve marked "Run 1" is the ratioof the two data sets on FIG. 11; the curve marked "Run 2" is the ratioof another, similar pair of data sets. Taken together, the two curvesyield a mean of R=0.878, and a standard deviation of σ_(R) =0.105. Inother words, on average only 12.2 percent of the available couplingefficiency is sacrificed in exchange for the advantages of batchalignment.

FIG. 11 and FIG. 12 represent results obtained prior to attachment ofthe substrate 3. Attaching the substrate is, of course, a potentialsource of further misalignment. However, tests show that its effect isrelatively minor. In FIG. 13, the coupled optical power corresponding tothe four bonded-out lasers is monitored before and then during thegluing and curing process used to attach the substrate. The left-mostset of four points on the graph represents the nominal value ofbatch-aligned coupled power (for the four energized lasers only) priorto disturbance by attachment of the substrate. The set of points secondfrom the left corresponds to the time at which the substrate just comesinto contact with the aligned array. The remaining points are taken atone-minute intervals during curing. The effects are mostly minor; netchanges in coupling efficiency are -4.0, -2.6, -0.9, and +9.4 percentfor the four lasers.

Further information is available after curing is complete, because thenall the lasers (not just the four bonded-out ones) may be energized, andthe coupled power emerging from each fiber may be compared to its valueprior to attaching the substrate. Such a comparison is shown in FIG. 14.The coupling efficiency for some laser/fiber pairs decreases during thegluing process, but increases for others. The increases are possiblebecause the original batch-aligned position is not optimal for everylaser/fiber pair, so that perturbation caused by attaching the substratemay move some pairs closer to optimal alignment while it moves otherpairs further away. On balance, as noted on FIG. 14, the mean efficiencyis about the same, while the standard deviation is slightly increased.In other words, attaching the substrate does not appear to be asignificant factor in degrading performance.

While the invention has been described in connection with specificembodiments, it will be understood that those with skill in the art maybe able to develop variations of the disclosed embodiments withoutdeparting from the spirit of the invention or the scope of the followingclaims:

What is claimed is:
 1. Apparatus for aligning a substantially co-linear array of lasers with a substantially co-linear array of optical fibers comprising;a. a single carrier having a slot for receiving both the array of lasers and the array of optical fibers; b. a first support plate having both a first holding means for holding said array of lasers in place with respect to said first support plate, and means for electrically coupling to selected ones of said lasers in said array of lasers; c. a second support plate having a second holding means for holding the array of fibers in rough alignment with the array of lasers; and d. means for positioning said carrier with respect to said first holding means and said second holding means so that when said holding means are activated, said array of lasers and said array of fibers are held in approximate alignment without said carrier.
 2. The apparatus of claim 1 wherein said holding means each comprise means for supplying a vacuum, and an opening in a respective support plate through which vacuum is applied to a respective array.
 3. The apparatus of claim 1 further comprising;a. adjustment means for moving said first support plate with respect to said second support plate; and b. an optical microscope for observing alignment of said array of lasers with respect to said array of fibers.
 4. The apparatus of claim 1 further comprising positioning means for positioning in close proximity to said array of lasers and said array of fibers, a substrate to which said array of lasers and said array of fibers are to be fastened.
 5. The apparatus of claim 4 wherein said positioning means includes a window positioned so that electromagnetic energy may be transmitted to said substrate.
 6. The apparatus of claim 5 wherein said window and said substrate are transparent to ultraviolet light so that ultraviolet curing adhesive applied to said substrate to fasten said substrate to said array of lasers and said array of optical fibers may be cured by said ultraviolet light.
 7. Apparatus for aligning a substantially co-linear array of lasers with a substantially co-linear array of optical fibers comprising;a. a carrier having a slot for receiving the array of lasers and the array of optical fibers; b. a first support plate having a first holding means for holding said array of lasers in place with respect to said first support plate, and means for electrically coupling to selected ones of said lasers in said array of lasers; c. a second support plate having a second holding means for holding the array of fibers in rough alignment with the array of lasers; d. means for moving said carrier between a first position wherein said array of lasers is spaced from said first holding means and said array of fibers is spaced from said second holding means to a second position in which said array of lasers is in contact with said first holding means and said array of fibers is in contact with said second holding means so that when said holding means are activated, said array of lasers and said array of fibers are held in approximate alignment when said carrier is moved away from said second position toward said first position.
 8. The apparatus of claim 7 wherein said holding means each comprise means for supplying a vacuum, and an opening in a respective support plate through which vacuum is applied to a respective array.
 9. The apparatus of claim 7 further comprising;a. adjustment means for moving said first support plate with respect to said second support plate; and b. an optical microscope for observing alignment of said array of lasers with respect to said array of fibers.
 10. The apparatus of claim 7 further comprising positioning means for positioning in close proximity to said array of lasers and said array of fibers, a substrate to which said array of lasers and said array of fibers are to be fastened.
 11. The apparatus of claim 10 wherein said positioning means includes a window positioned so that electromagnetic energy may be transmitted to said substrate.
 12. The apparatus of claim 11 wherein said window and said substrate are transparent to ultraviolet light so that ultraviolet curing adhesive applied to said substrate to fasten said substrate to said array of lasers and said array of optical fibers may be cured by said ultraviolet light.
 13. Apparatus for aligning a substantially co-linear array of lasers with a substantially co-linear array of optical fibers comprising;a. a carrier having a slot for receiving the array of lasers and the array of optical fibers; b. a first support plate having a first holding means for holding said array of lasers in place with respect to said first support plate, and means for electrically coupling to selected ones of said lasers in said array of lasers; c. a second support plate having a second holding means for holding the array of fibers in rough alignment with the array of lasers; d. means for positioning said carrier with respect to said first holding means and said second holding means so that when said holding means are activated, said array of lasers and said array of fibers are held in approximate alignment without said carrier; and e. positioning means for positioning in close proximity to said array of lasers and said array of fibers, a substrate to which said array of lasers and said array of fibers are to be fastened, said positioning means including a window positioned so that electromagnetic energy may be transmitted to said substrate, said window and said substrate being transparent to ultraviolet light so that ultraviolet curing adhesive applied to said substrate to fasten said substrate to said array of lasers and said array of optical fibers may be cured by said ultraviolet light.
 14. The apparatus of claim 13 wherein said holding means each comprise means for supplying a vacuum, and an opening in a respective support plate through which vacuum is applied to a respective array.
 15. The apparatus of claim 13 further comprising:a. adjustment means for moving said first support plate with respect to said second support plate; and b. an optical microscope for observing alignment of said array of lasers with respect to said array of fibers. 